librmpd is a ruby library for quickly creating mpd clients. The quickly part is due to the ability to register callbacks to the library for things such as song changes, playlist updates, etc. These callbacks can then update your client's interface with minimal programming required.
Several example clients are included in the library.
Also included in the library is a 'fake' mpd server. This server ONLY implements the client/server protocol, along with a fake database. This fake server is written in ruby, and is highly scriptable allowing for unit tests that can test your client under a known environment.
Note: This is still under development, so don't be surprised if things don't work. Just post a bug on rubyforge if things aren't working